Photosmart Premium C410b - Excessive Ink Consumption

I'm having an issue with my Photosmart C410b printer consuming excessive black ink. The last 364 XL cartridge lasted only 150 pages and the current 364 catridge lasted just 80! This printer serves my home office and it's crippling my business!

The printer is showing no errors. I've tried printing a diagnostic report - it appears normal apart from a smear of black ink where the 'black' print square should be. Cleaning the printheads does not improve the situation.

I'm at a loss - there is no advice on what to do on the HP main website. Urgent help appreciated to avoid having to rush out and buy a new printer from a different brand.